This invention relates to an image forming apparatus such as a digital copying machine or laser printer for scanning and exposing a single light beam or a plurality of light beams emitted from a semicon-ductor laser on a photosensitive drum surface to form a single electrostatic latent image on the photosensitive drum and more particularly to a light beam scanning apparatus provided on the image forming apparatus for scanning the single light beam or the plurality of light beams.
In recent years, various types of digital copying machines for forming an image by scanning and exposing the light beam and using the electrophotographic process are developed. Recently, in order to further enhance the image forming speed, a multi-beam type digital copying machine in which a plurality of light beams are emitted to simultaneously scan a plurality of lines by use of the plurality of light beams has been developed.
The multi-beam type digital copying machine includes an optical system unit as a light beam scanning apparatus having a plurality of semiconductor laser oscillators for emitting light beams, a polygonal rotating mirror such as a polygon mirror for reflecting the light beams output from the plurality of semicon-ductor laser oscillators towards the photosensitive drum and scanning the photosensitive drum by the light beams and a collimator lens and f-xcex8 lens as main components.
Conventionally, in the above multi-beam type digital copying machine, control of the exposure position of the light beam in the main scanning direction and control of the exposure position in the sub-the main scanning direction (that is, control of the passage position of the light beam) are effected in the optical system unit in order to form an image with high image quality.
A concrete example of the above technique is disclosed in, for example, Japanese Pat. Appln. KOKOKU Publication No. 1-43294, Japanese Pat. Appln. KOKOKU Publication No. 3-57452, Japanese Pat. Appln. KOKOKU Publication No. 3-57453, Japanese UM. Appln. KOKOKU Publication No. 5-32824, Japanese Pat. Appln. KOKAI Publication No. 7-72399, Japanese Pat. Appln. KOKAI Publication No. 7-228000, Japanese Pat. Appln. KOKAI Publication No. 9-210849, Japanese Pat. Appln. KOKAI Publication No. 9-258125, Japanese Pat. Appln. KOKAI Publication No. 9-314901 and Japanese Pat. Appln. KOKAI Publication No. 10-76704. However, the techniques disclosed in the above publications have the following problems.
That is, for control of the light beam exposure position in the main scanning direction, it is important to mount a light beam detecting device constructed by a plurality of optical sensors in a preset direction with respect to the main scanning direction of the light beam. That is, if the light beam detecting device is mounted in an inclined state, it becomes impossible to correctly detect the light beam position in the main scanning direction and, for example, there occurs a problem that a vertical line cannot be drawn straight.
However, an example indicating that the sensor itself has a function of detecting the relation between the mounting direction of the light beam detecting device and the main scanning direction of the light beam is disclosed only in Japanese Pat. Appln. KOKAI Publication No. 9-314901. Even in this example, the inclination detecting range is extremely narrow and there occurs a problem that detection and adjustment of the light beam position are difficult.
For control of the light beam position in the sub-scanning direction, examples in which the passage position of the light beam in the sub-scanning direction is replaced by time at which the light beam passes the sensor and detected are disclosed in Japanese Pat. Appln. KOKAI Publication No. 7-72399, Japanese Pat. Appln. KOKAI Publication No. 7-228000 and Japanese Pat. Appln. KOKAI Publication No. 9-210849.
However, if a variation occurs in the f-xcex8. characteristic of the f-xcex8 lens mounted on the optical system unit or a variation occurs in the rotation speed of the polygon mirror, then a variation will occur in the scanning speed of the light beam on the sensor and a detection error may occur when the detection method based on the passage time of the light beam is used.
Further, in Japanese Pat. Appln. KOKAI Publication No. 9-258125, Japanese Pat. Appln. KOKAI Publication No. 9-314901 and Japanese Pat. Appln. KOKAI Publication No. 10-76704, examples in which the passage position of the light beam is driven into a portion between specified sensor patterns formed on the light beam detecting device to set the passage position of the light beam in a preset position are shown. However, with this construction, it is necessary to independ-ently drive the light beams to the preset passage position and actuators for controlling the passage positions of the light beams are required by a number corresponding to the number of light beams. That is, in comparison with a case wherein one light beam is used as a reference and the passage positions of the remaining light beams are controlled, the number of actuators is larger by one and the cost becomes higher.
Further, if the detecting pattern for driving the light beam to the preset position is used, the precision of detection is high, but a range (detection range) in which each sensor output of the detecting pattern varies with a variation in the passage position of the light beam is narrow. Therefore, the control process becomes complicated and time for the control process becomes long.
If it is possible to control the passage position of each light beam for a plurality of resolutions, the number of sensor patterns for driving each light beam is increased and the structure of the sensor becomes complicated.
A first object of this invention is to provide a light beam scanning apparatus capable of enlarging the range (detection range) in which one sensor can respond to a variation in the passage position of a light beam, simplifying the control process and enhancing the control operation speed.
A second object of this invention is to provide a light beam scanning apparatus in which the number of actuators such as galvanomirrors for controlling the passage positions of the light beams is suppressed.
A third object of this invention is to provide a light beam scanning apparatus capable of coping with a plurality of resolutions with the simple sensor construction.
A fourth object of this invention is to provide a light beam scanning apparatus having a sensor for detecting the mounting inclination of the light beam detecting device with respect to the main scanning direction of the light beam in a wide range in the light beam detecting device.
A fifth object of this invention is to provide a light beam scanning apparatus capable of precisely detecting the passage position of the light beam irrespective of the scanning speed of the light beam on the sensor.
In order to achieve the above objects, according to one aspect of the present invention, there is provided a light beam scanning apparatus comprising: light beam emitting means for outputting a light beam; a beam scanner for reflecting the light beam output from the light beam emitting device towards a to-be-scanned surface to scan the to-be-scanned surface by use of the light beam in a main scanning direction; a first beam position detector for detecting the light beam scanned on the to-be-scanned surface by the beam scanner and generating an analog signal which is continuously changed with a variation in the passage position in a sub-scanning direction perpendicular to the main scanning direction of the light beam; and controller for controlling the position of the light beam scanned by the beam scanner on the to-be-scanned surface to a preset position based on the result of detection of the first beam position detector.
Further, according to this invention, a plurality of light beam emitting devices are provided and the beam scanner scans the to-be-scanned surface by use of a plurality of light beams emitted from the plurality of light beam emitting devices. The scanning apparatus further comprises light beam passage position changing means of a number smaller than the number of the plurality of light beam emitting devices by one, for changing the passage position of the light beam in the sub-scanning direction. The controller determines one of the plurality of light beams as a reference beam and changing the relative passage position of the remaining light beams with respect to the passage position of the reference light beam by use of the light beam passage position changing means.
Therefore, the number of actuators such as galvanomirrors for controlling the passage positions of the light beams can be suppressed. Further, the relative passage positions of the plurality of light beams can be precisely detected irrespective of the scanning speed of the light beam on the sensor.
According to another aspect of the present invention, there is provided a light beam scanning apparatus comprising: a plurality of light beam emitting devices for outputting light beams; a beam scanner for reflecting the light beams output from the light beam emitting devices towards a to-be-scanned surface to scan the to-be-scanned surface by use of the light beams in a main scanning direction; a first beam position detector for detecting the light beam scanned on the to-be-scanned surface by the beam scanner and generating an analog signal which is continuously changed with a variation in the passage position in a sub-scanning direction perpendicular to the main scanning direction of the light beam; a first target light detecting member having a first passage target and disposed separately from the first beam position detector in the main scanning direction; a second target light detecting member having a second passage target separated from the first passage target in the sub-scanning direction by a distance corresponding to preset resolution; light beam passage position changing means for changing the passage position of at least one of the plurality of light beams; and a controller for controlling the relation of the respective passage positions of the plurality of light beams to a preset relation by use of the light beam passage position changing means based on the outputs of the first beam position detector respectively obtained when the light beam has passed through the first and second passage targets.
There is further provided a light beam scanning apparatus the above, wherein the controller includes: calculating means for calculating a difference between the outputs of the first beam position detector respectively obtained when the light beam has passed through the first and second passage targets; and means for changing the passage position of one of first and second light beams among the plurality of light beams by use of the first beam passage position changing means to set the difference calculated by the calculating means equal to a difference between outputs of the beam position detector respectively obtained at the time of scanning by the first and second light beams.
There is further provided a light beam scanning apparatus the above, wherein the first beam position detector includes second and third beam position detectors; the second beam position detector generates an output which continuously decreases with a variation in the passage position of the light beam in the sub-scanning direction, the third beam position detector is disposed separately from the second beam position detector in the sub-scanning direction and generates an output which continuously increases with a variation in the passage position of the light beam, and the controller controls the passage position of the light beam to a preset position based on the results of detection of the second and third beam position detectors.
There is further provided a light beam scanning apparatus the above, further comprising: a fifth beam position detector disposed separately from the second and third beam position detectors in the main scanning direction, for detecting the light beam used for scanning the to-be-scanned surface by the beam scanner and generating an output which continuously decreases with a variation in the passage position of the light beam; a sixth beam position detector disposed adjacent to the fifth beam position detector in the sub-scanning direction, for detecting the light beam used for scanning the to-be-scanned surface by the beam scanner and generating an output which continuously increases with a variation in the passage position of the light beam; and inclination detecting means for detecting whole inclinations of the second to sixth beam position detectors with respect to the scanning direction of the light beam based on the results of detection of the second, third, fifth and sixth beam position detectors.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.